The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of Universal Mobile Telecommunication System (UMTS), and Long Term Evolution (LTE). LTE is also sometimes referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is a next generation wireless communication system relative to UMTS. LTE brings significant improvements in capacity and performance over previous radio access technologies.
The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and E-UTRAN is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a wireless device, also called a User Equipment (UE), is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to the UE and receiving signals transmitted by the UE. The area served by one or sometimes several RBSs may be referred to as a cell.
Wireless devices, which are referred to as UE in 3GPP terminology, may comprise, for example, cellular telephones, personal digital assistants, smart phones, laptop computers, handheld computers, machine-type communication/machine-to-machine (MTC/M2M) devices or other devices or terminals with wireless communication capabilities. Wireless devices may refer to terminals that are installed in fixed configurations, such as in certain machine-to-machine applications, as well as to portable devices, or devices installed in motor vehicles. Hereinafter, a wireless device may sometimes be referred to as a UE or simply as a device or terminal.
Location-based services and emergency call positioning drives the development of positioning in wireless networks and a plethora of applications and services in terminals take advantage of the position. Positioning in LTE is supported by the positioning architecture schematically illustrated in FIG. 1, with direct interactions between a UE and a location server, sometimes also referred to as an Evolved-Serving Mobile Location Centre (E-SMLC), via the LTE Positioning Protocol (LPP). The information transmitted between the location server and the UE will be handled by the eNodeB transparently, i.e., the eNodeB will control the transmission over the wireless link (LTE-Uu interface) to the UE but will not decode the actual information. Moreover, there are interactions between the location server and the eNodeB via the LPPa protocol, to some extent supported by interactions between the eNodeB and the UE via the Radio Resource Control (RRC) protocol. The E-SMLC is connected to the Mobility Management Entity (MME). An SLs interface is defined between the E-SMLC and MME and the MME is connected over the S1 interface to the eNB.
The following positioning techniques are considered in LTE (GPP TS 36.305 V13.0.0 (2015-12)):                Enhanced Cell ID: Essentially cell ID information to associate the UE to the serving area of a serving cell, and then additional information to determine a finer granularity position.        Assisted Global Navigation Satellite System (GNSS): GNSS information retrieved by the UE, supported by assistance information provided to the UE from E-SMLC.        Observed Time Difference of Arrival (OTDOA): The UE estimates the time difference of reference signals from different base stations and sends to the E-SMLC for multi-lateration.        Uplink Time Difference of Arrival (UTDOA): The UE is requested to transmit a specific waveform that is detected by multiple location measurement units (e.g. eNodeBs) at known positions. These measurements are forwarded to E-SM LC for multi-lateration        
The OTDOA is a UE-assisted method, in which the UE measures the time of arrival (TOA) of specific positioning reference signals (PRS) from multiple cells (or eNBs), and computes the relative differences between each cell and a reference cell. These reference signal time difference (RSTD) values are quantized and reported via LPP to the E-SMLC together with an accuracy assessment. Based on known positions of eNBs and their mutual time synchronization, it is possible for the E-SMLC to estimate the UE position from the RSTD values and covariance reports using multi-lateration. The accuracy depends on the radio conditions of the received signals, number of received signals, as well as the deployment, which means that the accuracy will vary spatially. FIG. 2 illustrates the multi-lateration in OTDOA when eNB1 is considered as the reference cell. For this case it is the measure of the relative difference between TOA of PRS from eNB3 and eNB1 (t3−t1) and the measure of the relative difference between TOA of PRS from eNB2 and eNB1 (t2−t1) that are relevant.
One of the factors which significantly impacts the performance of OTDOA, is the assumptions made for the UE receiver model and how it estimates the TOA.
How to Determine TOA
Wireless channels are usually modelled as multipath channels, meaning that the receiving node receives several distorted and delayed copies of the transmitted signal through multiple reflections and diffraction, etc. The multi-path effect can be modelled by considering the following tapped delay link channel.
      h    ⁡          (      t      )        =            ∑              l        =        0            L        ⁢                  ⁢                  a        l            ⁢              δ        ⁡                  (                      t            -                          τ              l                                )                    
L is the number of multipath taps, i.e., number of signals received at the UE, αl denotes the complex attenuation of the l-th tap (i.e., attenuation of the l-th signal received), τl indicates the time delay of the l-th tap and δ(t) is the delta function, which is one when t=0 and zero otherwise. In order to determine geographical distance between the transmitter and receiver antennas, one should measure τ0 which is the time delay corresponding to the line-of-sight (LOS) tap, and scale it with the speed of light.
TOA of the signal can be measured based on a reference signal that is known to the receiver. Assuming that the transmitted signal is denoted “x(t)”, then the received signal “y(t)” subject to multipath channel is given by
      y    ⁡          (      t      )        =                    ∑                  l          =          0                L            ⁢                          ⁢                        a          l                ⁢                  x          ⁡                      (                          t              -                              τ                l                                      )                                +          w      ⁡              (        t        )            
w(t) models additive noise and interference. Based on the received signal y(t) and the prior knowledge of the transmitted reference signal x(t), the receiver is interested in computing time delay of the first channel tap τ0, i.e., TOA of the LOS signal or the signal that arrives earliest if there is no LOS, since that translates to the distance between transmitter and receiver. However, since the received signal is embedded in noise and interference, it is not always easy to determine the first channel tap if it is not strong enough, which is usually the case in indoor scenarios.
There can be different methods to determine TOA at the receiver. A simple and widely used method is to cross-correlate the received signal with the known transmitted reference signal,
            R      ⁡              [        τ        ]              =                  ∑                  i          =          0                K            ⁢                          ⁢                        y          ⁡                      [            i            ]                          ⁢                              x            *                    ⁡                      [                          i              -              τ                        ]                                ,where K is the length of the received signal discrete domain representation. The cross-correlation function R(τ) gives channel impulse response. The absolute value of R(τ) corresponds to the Power Delay Profile (PDP) of the channel. The next step is to determine the first channel tap, which can be estimated by determining the first peak in R[τ] that is above a certain threshold ζ.
      τ    ^    =      argmin    ⁢                  {                                                                          R                ⁡                                  [                  τ                  ]                                                                                  max              ⁢                              {                                                    R                                                  }                                              ≥          ζ                }            .      
Finding the LOS component based on the cross-correlation as discussed above, is not an easy task for a UE. The UE needs to find a proper threshold in order to find the LOS component since the LOS tap is typically not the strongest tap. If the threshold is too low, the receiver can falsely detect noise as first channel tap and if the threshold is too high, the receiver may miss a weak LOS signal. Therefore, there is typically a trade-off between LOS detection and robustness to noise. In general, a higher SINR can improve the robustness to detect an LOS peak, that is, a higher SINR can mitigate the TOA estimation error caused by a strong non-LOS (N LOS) signal component.
For example, FIGS. 3a and 3b show situations where a UE fails to estimate a proper TOA when using a threshold-based peak detection. The graphs show how the cross-correlation values depend on the distance. The threshold is in the figures illustrated by a horizontal solid line (max peak/2). In FIG. 3a the leftmost vertical line (exact time) indicates the exact time (corresponding to a distance in meters) of the peak corresponding to the TOA of the LOS signal, and the rightmost vertical line (estimated) indicates the estimated time of the peak corresponding to the TOA of the LOS signal. In FIG. 3b it is the opposite and the leftmost vertical line (estimated time) indicates the estimated time of the peak corresponding to the TOA of the LOS signal, and the rightmost vertical line (exact time) indicates the exact time of the peak corresponding to the TOA of the LOS signal. These two example situations thus indicate some problems with a threshold-based peak detection. FIG. 3a exemplifies a situation where a lower threshold value would have improved the TOA estimation considerably. FIG. 3b exemplifies a situation where a higher threshold value would have improved the TOA estimation considerably.
PRS and PRS Configuration
In principle, it is possible to measure RSTD on any downlink signals, such as Cell-specific Reference Signals (CRS). However, in OTDOA the UE requires a detection of multiple neighbor-cell signals, and the CRS suffer from poor hearability and is therefore not suitable. Hence, PRSs have been introduced to improve OTDOA positioning performance. FIG. 4a and FIG. 4b illustrate the arrangement of the PRS assigned resources (black squares) for one resource block (RB) and for two different antenna port configurations using normal Cyclic Prefix (CP) and extended CP respectively. In such PRS subframes, no Physical Downlink Shared Channel (PDSCH) data is carried in order to reduce the interference with neighbor cells. Physical Downlink Control Channel (PDCCH) and CRSs are retained in the subframe, while PRSs are distributed in a “diagonal” way between CRSs. Alike CRS, a cell-specific frequency shift is applied to the PRS pattern. The number of frequency shifts is given by Physical Cell Identity (PCI) modulo 6, for avoiding time-frequency PRS collisions in up to six neighbor cells. Using a PRS pattern with a frequency shift given by PCI modulo 6 results in a frequency reuse factor of six. The frequency reuse factor is the rate at which the same PRS frequency can be used in the network. It is denoted K (sometimes 1/K) where K is the number of cells which cannot use the same frequencies for PRS transmission. A higher frequency reuse factor provides less time-frequency resource collisions for PRS, at the cost of less PRS resources per cell. Less PRS resources in a cell causes lower Signal to Interference and Noise Ratio (SINR).
The PRS sequence that is transmitted on antenna port 6 is created using the pseudo-random sequence generator that is initialized with:
            c      init        =                            2          10                ·                  (                                    7              ·                              (                                                      n                    s                                    +                  1                                )                                      +            l            +            1                    )                ·                  (                                    2              ·                              N                ID                cell                                      +            1                    )                    +              2        ·                  N          ID          cell                    +              N        CP                        N      CP        =          {                                    1                                              for              ⁢                                                          ⁢              normal              ⁢                                                          ⁢              CP                                                            0                                              for              ⁢                                                          ⁢              extended              ⁢                                                          ⁢              CP                                          
where NIDcell denotes the PCI of the Transmission Point (TP) that transmits the PRS, ns corresponds to the slot index within the radio frame, and l indexes the OFDM symbol within the slot.
New PRS Sequence Generation
For OTDOA, 3GPP has reached an agreement for a new PRS sequence and frequency shift (Vshift) generation per TP introduced in Rel.14. The pseudo-random sequence generator shall be initialized with:
      c    init    =                    2        28            ·              ⌊                              N            ID            PRS                    512                ⌋              +                  2        10            ·              (                              7            ·                          (                                                n                  s                                +                1                            )                                +          l          +          1                )            ·              (                              2            ·                          (                                                N                  ID                  PRS                                ⁢                mod                ⁢                                                                  ⁢                512                            )                                +          1                )              +          2      ·              (                              N            ID            PRS                    ⁢          mod          ⁢                                          ⁢          512                )              +          N      CP      where the quantity NIDPRS equals cell NIDcell unless configured otherwise by higher layers. NIDPRS is a value of a PRS identity, hereinafter referred to as prsID, which is a special identity of the PRS sequence as such, uncoupled from the cell (with a certain PCI corresponding to the cell NIDcell) that transmits the PRS sequence. In a similar way, the cell-specific frequency shift is given by vshift=NIDPRS mod 6, where the quantity NIDPRS equals NIDcell unless configured otherwise by higher layers. The range of the prsID values or NIDPRS is 0 to 4095. The new PRS sequence generation in Rel.14 is introduced to decouple the PRS from the PCI by introducing the prsID NIDPRS. This is performed in order to support more PRS sequences and to enable TPs with an associated macro cell in a shared cell identity scenario to be assigned with orthogonal PRS sequences. In a shared cell identity scenario, the same PCI is shared by multiple TPs. The current signaling proposal to enable the use of prsID with value NIDPRS is detailed in 3GPP R2-166548, “CR on OTDOA Enhancements for the Shared Cell-ID Scenario”. 
The IE OTDOA-ProvideAssistanceData is used by the location server to provide assistance data to enable UE assisted downlink OTDOA. One of the OTDOA Assistance Data Elements is the IE PRS-Info which provides the information related to the configuration of PRS in a cell. PRS-Info is according to the proposal extended as follows (new parts in bold):
-- ASN1STARTPRS-Info ::= SEQUENCE {prs-BandwidthENUMERATED { n6, n15, n25, n50, n75, n100, ... },prs-ConfigurationIndexINTEGER (0..4095),numDL-FramesENUMERATED {sf-1, sf-2, sf-4, sf-6, ...},...,prs-MutingInfo-r9CHOICE {po2-r9BIT STRING (SIZE(2)),po4-r9BIT STRING (SIZE(4)),po8-r9BIT STRING (SIZE(8)),po16-r9BIT STRING (SIZE(16)),...}OPTIONAL-- NeedOP [[ prsID-r14INTEGER (0..4095)OPTIONAL,--Need ON]]}-- ASN1STOP
Another of the OTDOA Assistance Data Elements is the IE OTDOA-ReferenceCellInfo which is used by the location server to provide assistance data related to reference cell information. The OTDOA-referenceCellInfo is extended with the field tpld which specifies the identity of a TP associated with a cell identified by a PCI (physCellId). The tpld field together with the physCellId and/or the prsID may be used to identify the TP when the same PCI is shared by multiple transmission points.
-- ASN1STARTOTDOA-ReferenceCellInfo ::= SEQUENCE {physCellIdINTEGER (0..503),cellGlobalIdECGIOPTIONAL,-- Need ONearfcnRefARFCN-ValueEUTRAOPTIONAL,-- CondNotSameAsServ0antennaPortConfigENUMERATED {ports1-or-2, ports4, ... }OPTIONAL,-- CondNotSameAsServ1cpLengthENUMERATED { normal, extended, ... },prsInfoPRS-InfoOPTIONAL,-- Cond PRS...,[[ earfcnRef-v9a0ARFCN-ValueEUTRA-v9a0OPTIONAL-- CondNotSameAsServ2]] ,[[ tpId-r14INTEGER (0..4095)OPTIONAL-- Need OR]]}-- ASN1STOP
The OTDOA Assistance Data Elements also comprises IE OTDOA-NeighbourCellInfoList used by the location server to provide neighbour cell information. The OTDOA-neighboringCellInfoElement is also extended with the tpld field as shown below:
-- ASN1STARTOTDOA-NeighbourCellInfoList ::= SEQUENCE (SIZE (1..maxFreqLayers)) OF OTDOA-NeighbourFreqInfoOTDOA-NeighbourFreqInfo ::= SEQUENCE (SIZE (1..24)) OF OTDOA-NeighbourCellInfoElementOTDOA-NeighbourCellInfoElement ::= SEQUENCE {physCellIdINTEGER (0..503),cellGloba1IdECGIOPTIONAL,-- Need ONearfcnARFCN-ValueEUTRAOPTIONAL,-- CondNotSameAsRef0cpLengthENUMERATED {normal, extended, ...}OPTIONAL,-- CondNotSameAsRef1prsInfoPRS-InfoOPTIONAL,-- CondNotSameAsRef2antennaPortConfigENUMERATED {ports-1-or-2, ports-4, ...}OPTIONAL,-- CondNotsameAsRef3slotNumberOffsetINTEGER (0..19)OPTIONAL,-- CondNotSameAsRef4prs-SubframeOffsetINTEGER (0..1279)OPTIONAL,-- CondInterFreqexpectedRSTDINTEGER (0..16383),expectedRSTD-UncertaintyINTEGER (0..1023),...,[[ earfcn-v9a0ARFCN-Va1ueEUTRA-v9a0OPTIONAL-- CondNotSameAsRef5]] ,[[ tpId-r14INTEGER (0..4095)OPTIONAL-- Need OR]]}
The current PRS design has a fixed frequency reuse factor of K=6 which is adequate, for example, when a UE is measuring RSTDs in a dense deployment. However, when considering sparsely deployed scenarios, or deep indoor scenarios such as MTC or Narrowband Internet of Things (NB-IoT) scenarios, where the UE is only capable of hearing a few number of cells, then the frequency reuse factor of K=6 results in an inefficient resource usage for PRS.